The main histopathological characteristic of Alzheimer's disease (“AD”) is the presence of neuritic plaques and tangles combined with associated inflammation in the brain. It is known that plaques are composed mainly of deposited (or insoluble in aqueous solution) fibrillar forms of the beta-amyloid (“A-beta”) peptide. The formation of fully fibrillar aggregated A-beta peptide is a complex process that is initiated by the cleavage of the amyloid precursor protein (“APP”). After cleavage of APP, the monomeric form of A-beta may associate with other monomers, presumably through hydrophobic interactions or domain swapping, to form dimers, trimers and higher-order oligomers. Oligomers of A-beta may further associate to form protofibrils and eventual fibrils, which is the main constituent of neuritic plaques. Soluble oligomers (soluble in aqueous buffer) of A-beta may contribute significantly to neuronal dysfunction. In fact, animal models suggest that simply lowering the amount of soluble A-beta peptide, without affecting the levels of A-beta in plaques, may be sufficient to improve cognitive function.
Presently, the only definitive method of AD diagnosis is postmortem examination of brain for the presence of plaques and tangles. The antemortem diagnosis of AD is difficult, especially during the early stages, as AD symptoms are shared among a spectrum of other dementias. Currently, AD diagnosis is achieved using simple cognitive tests designed to test a patient's mental capacity such as, for example, the ADAS-cog (Alzheimer's disease assessment scale—cognitive subscale) or MMSE (Mini-mental state examination). The subjective nature and inherent patient variability is a major shortcoming of diagnosing AD by such cognitive means. The fact that AD cannot be accurately diagnosed early creates a formidable challenge for pharmaceutical companies that aim to test anti-A-beta drugs as therapy to slow or halt AD pathogenesis. Furthermore, even if AD could be detected early and patients could be treated with A-beta lowering compounds, there is currently no way to know if the therapy is clinically efficacious. Therefore, a significant need exists to develop methods of measuring the soluble A-beta peptide levels locally in the brain.
Diagnosing AD by directly measuring levels of beta-amyloid non-invasively has been attempted by the targeted imaging of senile plaques. This approach fails as a specific measure of soluble A-beta peptide because current A-beta targeted imaging agents are directed at insoluble aggregates that are characteristic of A-beta fibrillar deposits in the brain. Small molecules that specifically bind to insoluble A-beta deposits include, for example, Congo red, Chrysamine G, methoxy-X04, TZDM, [11C]6, IMSB, Thioflavin(e) S and T, TZDM, 1-BTA, benzathiozole derivatives, [125I]3, BSB, IMSB, styrylbenzene-derivatives, IBOX, benzoxazole derivatives, IMPY, pyridine derivatives, DDNP, FDDNP, FENE, dialkylaminonaphthyl derivatives, and certain benzofuran derivatives (see, e.g., U.S. Pat. Nos. 6,133,259; 6,168,776; and 6,114,175).
Certain nucleic acid sequences have been shown to bind to insoluble senile plaques of A-beta, including mRNA for furin and amyloid precursor protein (“APP”).
Peptides also have been developed as imaging agents for insoluble deposits of A-beta and senile plaques. The sequence specific peptides that have been labeled for the purpose of imaging insoluble A-beta includes the labeled A-beta peptide itself, putrescine-gadolinium-A-beta peptide, radiolabeled A-beta, [111In]A-beta, [125I]A-beta, A-beta labeled with gamma emitting radioisotopes, A-beta-DTPA derivatives, radiolabeled putrescine, and KVLFF-based ligands.
Inhibitors of aggregated A-beta have been suggested to disrupt the formation of these aggregates by interacting with soluble or insoluble fibrils of A-beta. Examples of inhibitors or anti-aggregation agents include peptides of A-beta, KVLFF-based ligands, small molecular weight compounds, carbon nanostructures, rifamycin, IDOX, acridone, benzofuran, and apomorphine. Agents have also been identified that promote A-beta aggregation (e.g., agents such as A-beta42, proteins, metals, small molecular weight compounds, and lipids).
Targeted imaging of plaques may not provide early diagnosis, as large plaque burden is mostly associated with mid-to-late stage disease. Moreover, it has not been shown that current anti-A-beta therapies will affect fibrillar deposits appreciably to detect by imaging techniques at clinically relevant time points.
In vitro measures of A-beta may be specific for soluble A-beta in the cerebral spinal fluid, but lacks the necessary selectivity for local A-beta in the brain that is necessary for direct, accurate assessment of brain levels of soluble A-beta species. To date, the targeted non-invasive measurement and imaging of soluble A-beta peptide species that exist in the central nervous system have not been addressed.